Preheat oven to 425°F. Coat a large piece of heavy-duty foil with cooking spray; place potatoes on foil. Drizzle potatoes with water; wrap foil around potatoes, sealing tightly. Place foil packet in one corner of a 13 x 18–inch half sheet pan. Coat rest of sheet pan with cooking spray. Place panko, onion, 2½ tablespoons ketchup, ¼ teaspoon salt, ¼ teaspoon pepper, garlic, beef, and egg in a large bowl; mix well to combine. Add cheese; mix to combine. Divide mixture into 4 equal portions. Shape each portion into a 4 x 2½ –inch freeform loaf on sheet pan. Spread remaining 3 tablespoons ketchup evenly over meatloaves. Bake meatloaves and potatoes at 425°F for 15 minutes. Combine green beans and oil, tossing to coat. Remove pan from oven. Arrange green beans on pan around meat loaves and potatoes. Bake at 425°F for 15 minutes or until meatloaves register 165° on a thermometer. Remove pan from oven. Carefully open foil packet. Place potatoes in a bowl. Add milk, butter, ⅜ teaspoon salt, and ¼ teaspoon pepper. Mash with a potato masher to desired consistency. Sprinkle remaining ⅛ teaspoon salt and ¼ teaspoon pepper on green beans.